Theatre : CAVALCADE Sadler's Wells, London

For weeks now, the tom-toms have been beating to summon the long, the short, and the tall; the good, the bad, and the ugly; children of all ages; adults of every hue, for community service at Sadler's Wells. "With a cast of over 300" read the proud words on the front of Cavalcade's programme: not as impressive as the 400 in Noel Coward's 1931 production (every one of whom was a paid professional), but truly extraordinary in penny-pinching 1995. After its creaky start in Bromley, the show has now reached its London venue, where it plays for three weeks before sweeping west to Belfast, and then covering the North on its way to Glasgow for December.

Having caught Cavalcade in Brighton last month, where it was still pretty creaky, I have to report a brilliant transformation. Coward's family saga, which stretches from the Boer War to the start of the Thirties, is a ramshackle piece of writing in which the songs don't entirely paper over the cracks in the plot. And, since it's Coward, major textual liberties are forbidden. But, whereas in Brighton it actually felt ramshackle, it now runs swift and sure, a rumination on patriotism with a message for every age.

Cavalcade's huge supporting cast is new for each theatre it visits, which makes the achievement of Dan Crawford, its director and visionary begetter, all the more astonishing. His assistants have to leapfrog round the country inducting each new batch - and each new pair of sons for the principals - from a standing start in 24 hours. Yet somehow the whip has been cracked, so that the soldiers singing goodbye from the troop ship, the pleasure- seekers on the beach, and the crowds celebrating victory in Trafalgar Square now all radiate the right sort of razzmatazz.

To a simple but effective piano accompaniment, and on a set that transmutes effortlessly from water to dry land, from drawing room to railway station, history sweeps majestically along as Coward's tale unfolds. The trajectory of the First World War is conveyed with a sequence of songs that begins with "I'll Make a Man of You" and ends with "Roses of Picardy"; the cast first march up on stage in seemingly endless procession, and finally limp off in bandages.

Gabrielle Drake is the upstairs heroine who holds this world together, and Jeremy Clyde her gracefully ageing partner: demanding roles, played with impeccable style and conviction. The downstairs folk handle their stereotyped lines with brio, and the rich brats squabbling over their toy soldiers are genuinely comic. Penelope Woodman, playing a peachy ingenue, is one of the show's musical delights; Lisa Bowerman delicately intoning "If You Were the Only Girl in the World" and Caroline Faeber snarling her way through "20th Century Blues" are others. The boy soloists vary from sweet to excruciating, but that's just the luck of the draw.

The other - unexpected - hero of the day is the much-maligned theatre itself, shortly to be rebuilt from scratch. The stage may be too small for comfort, but Cavalcade shows off its good points: excellent acoustics (no need for mikes) and a lovely rapport between auditorium and stage. I have a nasty feeling in the pit of my stomach about what is soon to happen. The new architect's drawings look sensible - in a colourless Eighties way - and the building's original exterior is undeniably plain, but its bricks are rich in history. And, above all, it works.

n To 2 Sept. Box-office 0171-713 6000